US20040047757A1 - High-hardness, high-toughness steels and crawler components, earth wear resistant components, fastening bolts, high-toughness gears, high-toughness, high contact pressure resistance gears, and wear resistant steel plates using the same - Google Patents

High-hardness, high-toughness steels and crawler components, earth wear resistant components, fastening bolts, high-toughness gears, high-toughness, high contact pressure resistance gears, and wear resistant steel plates using the same Download PDF

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US20040047757A1
US20040047757A1 US10/391,732 US39173203A US2004047757A1 US 20040047757 A1 US20040047757 A1 US 20040047757A1 US 39173203 A US39173203 A US 39173203A US 2004047757 A1 US2004047757 A1 US 2004047757A1
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weight
steel
hardness
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Takemori Takayama
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Komatsu Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/011—Layered products comprising a layer of metal all layers being exclusively metallic all layers being formed of iron alloys or steels
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/22—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for drills; for milling cutters; for machine cutting tools
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/42—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for armour plate
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high-hardness, high-toughness, wear-resistible steel for use in an excavating edge member of a construction or earth work machine such as a hydraulic excavator, a bulldozer, a wheel loader, a motor grader, an underground piping burying machine, a soil-improvement machine, a crusher for concrete, lumber, or the like, and a tunneling machine; a crawler belt of a crawler vehicle; a reduction gear; and the like, and relates to a member using such a steel.
  • a construction or earth work machine such as a hydraulic excavator, a bulldozer, a wheel loader, a motor grader, an underground piping burying machine, a soil-improvement machine, a crusher for concrete, lumber, or the like, and a tunneling machine; a crawler belt of a crawler vehicle; a reduction gear; and the like, and relates to a member using such a steel.
  • examples of the wear-resistible steel widely used in construction or earth work machines include SMnB, SCr, SCrB, SCM, and SNCM medium carbon steels that are processed by heat-treatment such as quenching and tempering.
  • SMnB, SCr, SCrB, SCM, and SNCM medium carbon steels that are processed by heat-treatment such as quenching and tempering.
  • the components of a crawler vehicle belt such as a crawler bush, a crawler shoe, a crawler link, a tracker roller, and a sprocket are appropriately made wear-resistant based on the idea that toughness can be established by reducing the carbon content.
  • Excavating edge members for use in cutting or excavating rock mass (such as a ripper point, bucket tooth, and a cutting edge) are strongly needed to have high performance.
  • edge members have improved in toughness so that cracking or breakage can be prevented against more impact load and improved in wear resistance by high hardening.
  • the near-edge portion of the excavating edge member is heated to about 600° C. by severe friction with rock mass. Therefore, high-hardness, high-toughness steels with improved resistance to temper softening are often used for the edge member.
  • the construction or earth work machine in operation frequently goes over obstructions such as rock and other structures and frequently swings to excavate the obstructions.
  • the gears of the reduction gears in the driving and swinging mechanisms can be broken under impact load. Therefore, the gears are formed of low-carbon case hardened steel carburized, quenched, and tempered.
  • Toughness is important for the wear resistant members for forming the crawler belt. Therefore, the carbon content of such members is set at a low level, for example, as follows: 0.25 to 0.3% by weight in the crawler shoe, 0.3 to 0.35% by weight in the tracker roller, 0.35 to 0.4% by weight in the crawler link, and 0.35% by weight in the sprocket. In addition, their quenched and tempered hardness is adjusted to between HRC45 and HRC52. Under such conditions, the members are often insufficient in wear resistance and have a problem of the high cost of repairing the crawler belt in the construction or earth work machine.
  • the tooth plate of a jaw crusher for crushing rock and concrete is also toughness-conscious and often uses Hadfield steel. Such a plate also has a problem of insufficient wear resistance.
  • the edge portion of the excavating edge member such as a ripper point, bucket tooth, a cutting edge, and an end bit
  • the tunneling disk cutter is heated to a temperature of 300 to 600° C. by severe friction with the rock.
  • the initial hardness can significantly be reduced so that the wear resistance can be insufficient.
  • the steel to be used should have not only high toughness and high hardness but also sufficient resistance to temper softening even in heating at about 600° C.
  • Japanese Patent Publication No. 55-12177 (1980) discloses such a conventional wear-resistible steel, which contains C: 0.25 to 0.40%, Si: 1.5 to 2.5%, Mn: 1.6% or less, Cr: 3.0 to 5.0%, and Mo: 0.5 to 1.2%.
  • the high content of Si, Cr, or Mo can lead to sharp reduction in tempered hardness at 550° C. or higher.
  • Such a steel is insufficient in wear resistance and uneconomical.
  • Japanese Patent Laid-Open No. 59-107066 (1984) discloses a wear-resistible steel that contains C: 0.4 to 0.6%, Si: 1.6 to 2.2%, Mn: 0.5% or less, Cr: 1.0 to 1.5%, Mo: 0.8 to 1.2%, V: 0.2 to 0.5%, and Ni: 1.0 to 2.0%.
  • Such a wear-resistible steel is insufficient in toughness as well as resistance to temper softening because of the high content of Mo and V.
  • Japanese Patent Laid-Open No. 5-78781 (1993) discloses a wear-resistible steel that contains C: 0.35 to 0.55%, Si: 0.5% or less, Mn: 0.5% or less, P: 0.015% or less, S: 0.010% or less, Cr: 1.00 to 2.5%, Mo: 1.00 to 2.00%, V: 0.05 to 0.30%, B: 0.0003 to 0.0050%, Al: 0.005 to 0.10%, and Nb: 0.01 to 0.20%.
  • the low Si-induced reduction in the resistance to temper softening can be recovered by high Mo and V addition, but the recovered resistance to temper softening is not sufficient.
  • the content of P, S, and Mn is set low in the steel so that the grain boundary can be strengthened.
  • such a steel is generally expensive and therefore has a problem with commercial availability and can be insufficient even in toughness.
  • the gears In the driving and swinging mechanisms of the reduction gears of the construction or earth work machine, the gears must be prevented from fracturing due to impact load.
  • the gears are formed of high-toughness, low-carbon case hardening steels (with 0.1 to 0.25% by weight of C), carburized, quenched, and tempered.
  • the toughness can sharply decrease, as the surface carburized case becomes deeper.
  • the gears cannot improve in toughness, if the carburized case must be formed with a depth of 0.5 mm or more in terms of contact pressure strength and dedendum bending fatigue strength.
  • the low-carbon case hardening steels have also been reduced in P and S contents to have strengthened grain boundary, but such steels are generally expensive and therefore have a problem with commercial availability and can be insufficient even in toughness.
  • the present invention has been made in order to solve these problems. It is therefore an object of the present invention to provide a high-hardness, high-toughness steel that contains Si, Al, Cr, Mo, V, W, Ni, and Co more appropriately added so as to have an HRC hardness of 50 or higher and a Charpy impact value of 5 kgf m/cm 2 or more by tempering at a high temperature of 600° C. or higher.
  • the inventor has paid attention to a commonality between the fact that the excavating edge member or the like is heated up to 600° C. by friction and the process in which quenched steels are tempered at 550° C. or higher to recover the toughness without temper brittleness.
  • the inventor has investigated and found appropriate carbon content for providing a sufficient hardness of HRC45 or higher (preferably HRC50 or higher) even after quenching and tempering at 550° C.
  • alloying elements such as C, Si, Al, Cr, Mo, V, and W
  • appropriate technique in consideration of the interaction between alloying elements as described below so that unnecessary addition of the alloying elements, which would otherwise cause brittleness and deterioration, can be prevented, and economy can be achieved.
  • Co which can raise the magnetic transformation temperature of quenched and tempered martensite, is added in an appropriate amount so that high toughness is produced without high temperature temper brittleness, and therefore the resistance to high temperature temper softening further increases without reduction in toughness.
  • the high-hardness, high-toughness steel for use in the member for which resistance to temper softening is important contains 0.25 to 0.55% by weight of carbon.
  • the carbon content has little effect on the hardness during tempering at 400 to 600° C. (a carbon content of 0.55% by weight or more provides +2.5 of ⁇ HRC at 500° C. and +1.0 of ⁇ HRC at 600° C.). Therefore, analysis was carried out using, as the amount of resistance to temper softening (an increase in hardness), the difference between the hardness obtained by adding alloying elements and by tempering at 400 to 600° C. and the standard hardness of carbon steel. In order to obtain a hardness of HRC45 or higher by tempering at 600° C., the carbon content is preferably 0.25% by weight or more.
  • an alloying element such as Mo and V is positively added to the high-hardness, high-toughness steel.
  • a carbon content of 0.60% by weight or more is not preferred. This is because such an increased carbon content can reduce the solid solution amount of the alloying element which would otherwise contribute to the resistance to temper softening in the austenite phase area during heating for quenching; the reduction in the role of the alloying element contributing to the resistance to temper softening is not economical; and the carbide in the tempered martensite increases in amount and size so that the toughness is reduced.
  • the carbon element can significantly stabilize the austenite.
  • the inventive steel that contains a large amount of ferrite-stabilizing Si, Al, or Mo preferably contains 0.10% by weight or more of carbon so that the quenching temperature can be suppressed to 950° C. or lower.
  • the carbon content is preferably 0.10% by weight or more.
  • Si is inevitably introduced by steel making and the Si content is generally 0.05 to 0.3% by weight.
  • Si may be added in an amount of less than 2.5% by weight, because it can suppress the precipitation of cementite, contribute to the improvement in toughness by tempering at about 400° C. or lower, and enhance the resistance to temper softening.
  • a Si addition amount of less than 0.3% by weight cannot provide such significant effects.
  • a Si addition amount of up to about 4% by weight is known to enhance the resistance to temper softening.
  • the addition amount of Si should be determined in such a range that Si can stabilize the ⁇ Fe phase to raise the A3 transformation temperature and does not excessively raise the quenching temperature.
  • the Si addition amount is preferably suppressed to 2.5% by weight or less and is more preferably 1.8% by weight in terms of the effect of Si addition on Mo or V as described below.
  • Si and V have also been found to interact with each other similarly to Si and Mo.
  • the effective maximum addition amount of V (YV) was 0.15% by weight at 925° C., and at less than 1.8% by weight of Si, it was 0.3% by weight, and at 950° C., such maximum addition amounts were 0.2% by weight and 0.4% by weight, respectively.
  • the steel also contains Al, which can stabilize the ferrite phase of the steel similarly to Si. Therefore, the total addition amount of Al and Si should be 1.8% by weight or less (Al+Si ⁇ 1.8% by weight) so that an excessive raise in the quenching temperature can be avoided.
  • the coexisting Cr at a content of 3.5% by weight or more also reduces the upper limit of the Mo addition amount to about half, otherwise such Mo would be effective at enhancing the resistance to temper softening. Therefore, it is apparent that Mo in the effective addition amount or more can reduce the toughness.
  • Al has a very strong deoxidizing action. It is known that Al reacts with nitrogen in the steel to form AlN and make the crystal grains fine. Killed case hardened steels generally contain 0.005 to 0.05% by weight of Al. The solid Al dissolved in the steel has a strong tendency to segregate at the grain boundary and functions to strongly exclude, from the grain boundary, the impurity element such as P and S, which can reduce the grain boundary strength, and to strongly attract Ni, which can improve the grain boundary toughness. According to the present invention, therefore, Al and Ni are positively added at the same time so that the carbon content can be high and the tempered martensite structure steel with a high hardness of HRC45 or higher can drastically improve in toughness.
  • Al coexistent with Ni is preferred in terms of wear resistance, because in such a case, the development of the age hardening can further enhance the resistance to temper softening (1Al-1Ni can produce +4 of ⁇ HRC at 600° C.) as described below.
  • Ni 0.3 to 2.5% by Weight
  • the combined addition of 0.3 to 2.0% by weight of Al and Ni is essential for more effective contribution to toughness improvement, and therefore the lower limit of the Ni addition amount should be 0.3% by weight.
  • the upper limit of the Ni addition amount is preferably 2.5% by weight, because the combined addition of Ni and Al can enhance the resistance to temper softening by the precipitation of NiAl intermetallic compounds and improve the wear resistance, but excessive addition can reduce the toughness and be economically disadvantageous.
  • FIG. 1 shows that based on the effect of various alloying elements on the A3 temperature line of Fe-3 wt % Si—C alloy, alloyed steels with a lower carbon content (from 0.10% by weight) can preferably be reduced in the heat treatment cost by the quenching temperature-lowering effect of the Mn, Ni, or Cr addition, and even at a C content of about 0.4% by weight, Mn or Ni may preferably be added in an amount of about 1% by weight.
  • Mn has a significant desulfurizing action. Mn is effective at improving the hardenability of steels. Similarly to Ni, Mn can strongly stabilize the austenite phase of steel so that the A3 transformation temperature can be lowered and the quenching temperature can effectively be reduced.
  • the Mn element is also effective at suppressing the raise in the A3 transformation temperature by the addition of the ferrite-stabilizing element Al or Si. In the present invention, therefore, the Si addition amount should be 3.0% by weight or less, considering the effect of Mn, Ni, Si, and Al on the eutectoid temperature and the approximate relation: (Si+2 ⁇ Al) ⁇ (Ni+Mn).
  • the quenching temperature is preferably suppressed not to be 950° C. or higher, and the old austenite crystal grain is preferably suppressed in growth not to have an ASTM grain size number of greater than 8, in terms of heat treatment cost.
  • Cr can improve the hardenability of steels and enhance the resistance to temper softening. Its hardening effect is, however, smaller than that of Mo, V, W, or the like. If its addition amount is more than about 7.5 times as much as the coexisting carbon content, the resistance to temper softening per Cr addition amount (% by weight) at higher temperature can be reduced and the Si or Al-induced resistance to temper softening at higher temperature can also be reduced. Therefore, if the steel should benefit from the Si or Al-induced resistance to temper softening, the maximum of the Cr addition amount should be at most 7.5 times as much as the carbon content.
  • [0051] 2 A method in which the Cr addition amount is set in the range of 3.5 to 5.5% by weight, and then the Si addition amount is set at 0.5% by weight or less, and Mo is set in the effective addition amount range (up to 1.0% by weight), and Al, V, and W are controlled (a Mo addition amount of 1.0% by weight or more cannot effectively contribute to the enhancement of the resistance to temper softening).
  • Si is preferably added in a small amount
  • Mo is preferably added in an amount of the effective addition amount or less
  • V or W is preferably added to enhance the resistance to temper softening. For example, in order to make the 600° C.
  • the elements are preferably added as follows: Si: 0.5% by weight or less, Cr: 3.5 to 5.5% by weight, Mo: 0.3 to 1.0% by weight, V: 0.2 to 0.4% by weight, and W: 0.1 to 0.8% by weight, and optionally Al: 0.15 to 0.6% by weight and Ni: 0.3 to 1.5% by weight.
  • Mo can improve the hardenability. Mo can also enhance the toughness of the low-temperature tempered martensite steel and the resistance to temper softening as mentioned above. Therefore, the lower limit should be 0.1% by weight for the development of an effective resistance to temper softening, and the upper limit should be the maximum addition amount (YMo % by weight) up to which Mo can be effective for the resistance to quench softening at quenching temperature.
  • the effective maximum addition amount (YMo % by weight) is further reduced to about half.
  • the content of Mo is preferably 1% by weight or less.
  • V 0.05 to 0.4% by Weight
  • V can significantly enhance the resistance to temper softening in the tempering temperature range of 600° C. or higher and be effective at improving wear resistance.
  • V or W W has a V-like action as described below
  • the solid solubility of V carbide is low, and the V carbide can precipitate into the austenite phase during heating at quenching temperature so that the toughness can be reduced. Therefore, the upper limit of the V addition amount is preferably 0.3% by weight. As described above, the upper limit can appropriately be changed into 0.4% by weight at a quenching temperature of 950° C. and 0.5% by weight at 1000° C.
  • the upper limit of the V addition amount should be reduced to half, that is, 0.15, 0.2, or 0.25 wt % at each quenching temperature.
  • the quenching temperature is preferably 950° C. or lower in terms of quenching equipment and the productivity thereof and coarse crystal grain grown by heating. Therefore, the upper limit of the addition amount should be 0.4% by weight. More preferably, the quenching temperature is 900° C. or lower, and therefore the upper limit of the addition amount is 0.3% by weight.
  • a V addition amount of 1.0% by weight or more can reduce the Si-induced resistance to temper softening, and therefore the maximum addition amount is preferably 0.4% by weight or less.
  • W does not produce resistance to temper softening as much as Mo or V does.
  • the resistance to temper softening produced by W can be maximum at a temperature of 600 to 700° C., and the upper limit of the addition amount up to which W is effective (YM) can be high. Therefore, either W or V is an essential component.
  • W can be effective in an addition amount of 0.1% by weight or more.
  • the effective upper limit depends on temperature and should be 0.8% by weight at 900° C., 1.7% by weight at 950° C., and 2.5% by weight at 1000° C. If W is added in an amount of 1% by weight or more, the Mo-induced resistance to temper softening can significantly reduced. W is more expensive than Mo. Therefore, the maximum addition amount is preferably 1.0% by weight or less.
  • Co itself does not produce resistance to temper softening.
  • the addition of Co can sharply raise the magnetic transformation temperature and suppress the diffusion of other alloying elements.
  • Co can be effective at raising the reaction temperature in the formation of Si, Al, Cr, Mo, V, or W carbide which can produce the resistance to temper softening.
  • Co can exert age hardening together with Si and Al, and coexisting Al can efficiently raise the Co magnetic transformation temperature.
  • the magnetic transformation temperature increased by 18° C. per 1% by weight, and such an effect was obtained up to 10% by weight.
  • an increase of 10° C. was obtained per 1% by weight.
  • the effect will be saturated, and the cost can be too high.
  • the usage of Co is preferably 10% by weight or less.
  • B can significantly improve hardenability.
  • the addition of B can be economically advantageous, because the usage of the hardenability-enhancing alloying element such as Mn, Cr, and Mo can be reduced.
  • the addition amount of Cr which tends to cause high-temperature temper brittleness, can be reduced. In the present invention, therefore, positive use of B is preferred.
  • the addition amount of B is appropriately from 0.0005 to 0.0030% by weight, because it cannot be effective in an amount of less than 0.0005% by weight, and an amount of more than 0.0030% by weight is known to produce BN precipitation that can reduce the toughness.
  • Zr, Nb, and Ti are known to make crystal grains fine and added in a conventional amount range.
  • an amount of more than 0.2% by weight is known to increase the amount of carbide and nitride precipitations and to be disadvantageous for toughness.
  • P and S are contained as inevitable impurities but important because they are involved in the temper brittleness at a temperature of 350 to 550° C.
  • the content of these elements in high cleanliness steels is reduced as low as possible.
  • the maximum content of P or S may be more than 0.03% by weight, because high temperature tempering at 600° C. or higher can be used and/or the addition of Al and Ni can prevent the temper brittleness.
  • 0.03% by weight or less is preferred in terms of stabilization of higher toughness, and 0.015% by weight or less is more preferred, because such an amount presents no cost problem with the conventional steel making technique.
  • the present invention is based on the above discussion and results.
  • the high-hardness, high-toughness steel contains Mo, V, and W in such an appropriate addition amount that they can produce strong resistance to temper softening while efficiently using Si-induced resistance to temper softening.
  • Such a steel is economical and highly tough and wear-resistant.
  • the Cr addition amount is limited to less than 1% by weight, the amounts are limited as follows: Si: 0.8-1.6% by weight, Cr: 0.1-1.0% by weight, and Mo: 0.5-1.3% by weight, and B is added in an amount of 0.0005 to 0.005% by weight.
  • the addition amount of each alloying element is controlled to satisfy the relation formula: 26.2 ⁇ 5.8 ⁇ (Si(% by weight)+Al(% by weight))+2.8 ⁇ Cr(% by weight)+11 ⁇ Mo(% by weight)+25.7 ⁇ V(% by weight)+7.5 ⁇ W(% by weight) ⁇ 36.2, and quenching is carried out from a temperature of 950° C. or lower and then tempering process is carried out at 600° C. so that an HRC hardness of 50 to 60 is provided.
  • the high-hardness, high-toughness steel contains at least 0.1 to 1.20% by weight of carbon and 0.05 to 1.8% by weight of Si and is characterized in that Si is partially replaced by 0.15 to 1.6% by weight of Al, Ni is added to the steel in an amount of 0.3 to 2.5% by weight, the steel contains at least one alloying element of Mn, Cr, Mo, V, W, Co, Cu, Ti, B, Nb, Zr, Ta, Hf, and Ca; and inevitable impurities including P, S, N, and O, the balance consists essentially of Fe, and the steel has a quenched and tempered martensite structure.
  • the Cr content is preferably in the range of 0.1 to 3.5% by weight.
  • One or both of 0.05 to 0.40% by weight of V and 0.1 to 1.0% by weight of W are preferably added so that the resistance to temper softening can further be enhanced.
  • Al is in an amount of 0.15 to 0.75% by weight in order to prevent excessive Al addition and give an adequate increase in the A3 transformation temperature, and one or both of limiting the Ni amount to between 0.3 and 2.0% by weight and adding 0.0005 to 0.005% by weight of B for enhancement of hardenability at low cost are made.
  • the addition amount of each alloying element is preferably controlled to satisfy the relation formula: 21.2 ⁇ 5.8 ⁇ (Si(% by weight)+Al(% by weight))+2.8 ⁇ Cr(% by weight)+11 ⁇ Mo(% by weight)+25.7 ⁇ V(% by weight)+7.5 ⁇ W(% by weight) ⁇ 41.2.
  • the high-hardness, high-toughness steel contains at least C: 0.25 to 0.55% by weight, Si: less than 0.8% by weight, and Cr: 3.5 to 5.5% by weight and is characterized in that Mo is added to the steel in an amount of 0.3 to 1.0% by weight, one or both of V: 0.10 to 0.40% by weight and W: 0.1 to 1.0% by weight are added to the steel, the steel contains at least one alloying element of Mn, Ni, Co, Cu, Al, B, Ti, Nb, Zr, Ta, Hf, and Ca; and inevitable impurities including P, S, N, and O, the balance consists essentially of Fe, and the steel is a tempered martensite steel.
  • the addition amount of each alloying element is controlled to satisfy the relation formula: 21.2 ⁇ 3 ⁇ (Si(% by weight)+Al(% by weight))+2.8 ⁇ Cr(% by weight)+11 ⁇ Mo(% by weight)+25.7 ⁇ V(% by weight)+7.5 ⁇ W(% by weight) ⁇ 41.2 in order to ensure an HRC hardness of 45 or higher by tempering at 600° C.
  • Co may be added in an amount of 1 to 20% by weight so that the magnetic transformation temperature of the tempered martensite can increase by about 200° C., the diffusion of alloys during the tempering process can significantly be slowed down, and therefore significant resistance to softening can be developed by tempering even at 600° C. or higher.
  • At least one of Nb, Ti, Zr, Ta, and Hf is preferably added in a total amount of 0.005 to 0.2% by weight in order to produce fine crystal grains in heating at high temperature for quenching.
  • the high-hardness, high-toughness steel quenched and then tempered at a high temperature of 600° C. or higher can have an HRC hardness of 50 or higher and a Charpy impact value of 5 kgf m/cm 2 or more.
  • the steel quenched and then tempered at a temperature of 150° C. or higher can have an adjusted HRC hardness of 45 or higher and a Charpy impact value that satisfies the relation formula: log(Charpy impact value (kgf m/cm 2 )) ⁇ 0.0263 ⁇ HRC+2.225 where its HRC hardness is in the range from 45 to 55, or have a HRC hardness of 55 or higher and a Charpy impact value of 6 kgf m/cm 2 or more where its HRC hardness is 55 or higher.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may be processed into a crawler component such as a crawler bush, a crawler link, a top or bottom tracker roller for a crawler, and a crawler shoe, characterized by having a quenched and tempered HRC hardness of 52 or higher and a Charpy impact value of 6 kgf m/cm 2 or more and comprising a wear-resistant portion whose wear resistance is increased to at least 1.2 times on average as much as that of a conventional crawler component.
  • a crawler component such as a crawler bush, a crawler link, a top or bottom tracker roller for a crawler, and a crawler shoe, characterized by having a quenched and tempered HRC hardness of 52 or higher and a Charpy impact value of 6 kgf m/cm 2 or more and comprising a wear-resistant portion whose wear resistance is increased to at least 1.2 times on average as much as that of a conventional crawl
  • Such a component can be obtained through whole heating, quenching with a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil, and tempering at a temperature of 150° C. to 400° C.
  • a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil
  • tempering at a temperature of 150° C. to 400° C.
  • the wear-resistant portion of each component may be formed through high-frequency heating for quenching and tempering.
  • the through-hardened crawler bush or link can maintain a high HRC hardness of 55 or higher even at a tempering temperature of 400° C. or lower and show a very high Charpy impact value, for example, even where it contains 0.6% by weight of carbon, 1% by weight of Al and 1% by weight of Ni (see FIG. 11). Therefore, it will be understood that the crawler bush and the crawler link in the structure of the crawler bush press-fitted into the crawler link can improve in the resistance to delayed fracture. Therefore, the cost of heat-treating these components can considerably be lower than that of heat-treating the crawler bush produced by a conventional carburizing, quenching, and tempering process or a conventional inside-outside diameter induction hardening process.
  • Such low-cost heat-treatment can be used in place of deep induction hardening of the tread portion of the crawler link to the bottom tracker roller after thermal refining of the material. It will also be understood that chipping at both ends of the crawler link which collide with the bottom tracker roller can be prevented.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may be processed into an earth wear-resistant component such as a tunneling shank, a tunneling disk cutter, a chisel tool, and a stirring blade for soil improvement, characterized by having a quenched and tempered HRC hardness of 50 or higher and a Charpy impact value of 8 kgf m/cm 2 or more, wherein the hardness and the Charpy impact value are each increased to at least 1.2 times on average as much as that of a conventional component.
  • an earth wear-resistant component such as a tunneling shank, a tunneling disk cutter, a chisel tool, and a stirring blade for soil improvement
  • Such a component can be obtained through whole heating, quenching with a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil, and tempering at a temperature of 150° C. to 350° C.
  • a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil
  • tempering at a temperature of 150° C. to 350° C.
  • the wear-resistant portion of each component may be formed through high-frequency heating for quenching and tempering.
  • the addition amount of Ni is preferably 1% by weight.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may be processed into a fastening bolt for use in construction equipment such as a crawler shoe bolt, a reduction gear, swing circle fixing bolts, characterized by having a carbon content of 0.30% by weight or more, a quenched and tempered HRC hardness of 40 or higher, and a Charpy impact value that satisfies the relation formula: log(Charpy impact value (kgf m/cm 2 )) ⁇ 0.0263 ⁇ HRC+2.225.
  • Such a component can be obtained through whole heating, quenching with a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil, and tempering at a temperature of 150° C. to 500° C.
  • a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil
  • tempering at a temperature of 150° C. to 500° C.
  • the thread portion of the bolt may be induction-hardened and tempered.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may be formed into a gear shape and then carburized, quenched, and tempered to result in a gear or a high-toughness gear such as a crawler bush and a crawler shoe bolt, characterized by having a surface carbon concentration of 0.6 to 1.0% by weight, a surface carburizing depth of 0.4 mm or more, and an adjusted HRC hardness of 55 to 64, and providing a Charpy test piece with the equivalent depth and a Charpy impact value of 5 kgf m/cm 2 or more, preferably 8 kgf m/cm 2 or more.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may be formed into a gear shape, and then carburized to have a surface carbon content of 0.8 to 1.3% by weight, temporarily cooled down to the Al transformation temperature or lower, and then heated again, quenched, and tempered to result in a high-toughness, high contact pressure-resistance gear, characterized by having a surface carburized case depth of 0.4 mm or more, containing cementite particles with an average particle diameter of 1 ⁇ m or less dispersed in its quench-hardened case, having an adjusted HRC hardness of 59 to 65, and providing a Charpy test piece with its equivalent depth and a Charpy impact value of 4 kgf m/cm 2 or more.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may be formed into a gear shape and then induction-hardened and tempered to result in a high-toughness gear, characterized by having an adjusted surface HRC hardness of 52 to 64 and providing a Charpy test piece with its equivalent hardened depth and a Charpy impact value of 5 kgf m/cm 2 or more.
  • the gear can be obtained through whole heating, quenching with a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil, and tempering at a temperature of 150° C. to 350° C.
  • a suitable cooling medium such as water, an aqueous quenching solution, and a quenching oil
  • the gear tooth portion may be induction-hardened and tempered to have a Charpy impact value of 5 kgf m/cm 2 or more.
  • the high-hardness, high-toughness steel according to each above aspect of the present invention may quenched and tempered to result in a wear-resistant steel plate, characterized by having a high tension of 50 kgf/mm 2 or higher and/or an adjusted HRC hardness of 50 or higher and being weldable to a bucket, a bulldozer blade, or the like for use.
  • the wear-resistant steel plate can improve in low-temperature cracking susceptibility in welding to a bucket, a blade or the like of a construction or earth work machine or improve in cracking susceptibility in re-heating.
  • the steel according to each above aspect of the present invention is applicable to excavating edge members that set importance on the resistance to temper softening under frictional heating. Therefore, the present invention is also directed to an earth wear-resistant component for use in earth excavation such as a ripper point, an end bit, bucket tooth, an edge, and a tunneling disk cutter, characterized by comprising the high-hardness, high-toughness steel which contains less than 3.5% by weight of Cr and alloying elements each in a controlled addition amount that satisfies the relation formula: 26.2 ⁇ 5.8 ⁇ (Si(% by weight)+Al(% by weight))+2.8 ⁇ Cr(% by weight)+11 ⁇ Mo(% by weight)+25.7 ⁇ V(% by weight)+7.5 ⁇ W(% by weight) ⁇ 41.2 so that the steel can have an HRC hardness of 50 or higher by tempering at 600° C.
  • an earth wear-resistant component for use in earth excavation such as a ripper point, an end bit, bucket tooth, an edge, and a tunnel
  • FIG. 1 is a graph showing the effect of various alloying elements on the Ac3 temperature lines of Fe—Si based alloys
  • FIG. 2 is a graph showing tempered hardness of various wear-resistant steels
  • FIG. 3 is a first graph showing the relationship between hardness of various tempered steels and impact values thereof;
  • FIG. 4 is a second graph showing the relationship between hardness of various tempered steels and impact values thereof;
  • FIG. 5 is a graph showing the results of preliminary experiments on the relationship between the Charpy impact value and the carbon content
  • FIG. 6 is a graph showing found values (at a quenching temperature of 870° C.) and calculated values of tempered hardness on products Nos. 1 to 9 for comparison;
  • FIG. 7 is a graph showing found values (at a quenching temperature of 870° C.) and calculated values of tempered hardness on products Nos. 10 to 22 for comparison;
  • FIG. 8 is a graph showing found values (at a quenching temperature of 900° C.) and calculated values of tempered hardness on products Nos. 23 to 29 for comparison;
  • FIG. 9 is a graph showing found values (at a quenching temperature of 950° C.) and calculated values of tempered hardness on products Nos. 30 to 33 for comparison;
  • FIG. 10 is a graph showing found values (at a quenching temperature of 900° C.) and calculated values of tempered hardness on products Nos. 34 to 38 for comparison;
  • FIG. 11 is a graph showing the relationship between the tempered hardness and the Charpy impact value on products Nos. 47 to 49 ;
  • FIG. 12 is a graph showing the relationship between the tempering temperature and the Charpy impact value on products Nos. 10 , 12 , and 47 ;
  • FIG. 13 is a graph showing conditions of carburizing and quenching
  • FIG. 14 is a diagram showing hardness distribution in carburized, quenched, and tempered Charpy test pieces
  • FIGS. 15 ( a ) and 15 ( b ) are micrographs each showing a surface quench-hardened layer structure formed through the steps of carburizing at 1000° C. for 2 hours to provide a surface carbon concentration of 1.1% by weight or 1.3% by weight, cooling to room temperature, and re-heating at 850° C. for 1 hour for quenching and tempering; and
  • FIG. 16 is a graph showing the relationship between hardness and wear ratio on various steels where the gouging wear amount of the quenched and tempered S45C steel (Hv 500) is normalized as 1.
  • Example 1 the handbook “Hagane no Netsu-Shori” (in Japanese) (Heat-Treatment of Steels), revised 5th edition, edited by The Iron and Steel Institute of Japan, published by MARUZEN CO., LTD, 1985 was referred to, and the relations between tempering temperature and Rockwell hardness on various tough steels (martensite steels) described therein were organized so that target values for improvement of wear-resistant steels were investigated in making the present invention.
  • FIG. 3 and 4 shows the relationship between the 150 to 700° C. tempered hardness and the Charpy impact value on steels including SUJ2 and SKH9. These graphs show that the Charpy impact value can be 5 kgf m/cm 2 or more where the HRC hardness has an upper limit of about 56.
  • FIG. 5 shows the result of preliminary experiments in which steels each having the composition as shown in Table 1 were quenched and then tempered at 200° C. for 2 hours, and the relationship between the Charpy impact value and the carbon content was examined. The result shows that there is almost no possibility that a carbon content of 0.55% or more provides a Charpy impact value of 5 kgf m/cm 2 or more. TABLE 1 TPNo.
  • Table 2 shows the compositions of the steels used.
  • the addition amount of each element was in the following range: C: 0.14 to 0.73% by weight, Si: at most 2.5% by weight, Mn: at most 1.3% by weight, Cr: 0.3 to 8% weight, Mo: at most 4% by weight, V: at most 1% by weight, W: at 2% by weight, Al: at most 2% by weight, and Ni: at most 2% by weight.
  • Other elements such as Nb, B, and Ti were added in a very small amount, and each level was selected concerning the control of P and S and the like.
  • the steels were used in investigating the effect of each alloying element on the resistance to temper softening (and the Charpy impact value).
  • Each ingot steel about 25 kg in weight was prepared using a high-frequency smelter, formed into a round bar shape 32 mm in diameter by hot forging, machined into a round bar 25 mm in diameter, cut to have a suitable length, heat-treated in a certain manner, and subjected to the experiments as shown below.
  • TABLE 2 Quenching Hardness Hardness Temperature HRC Charpy Impact Value
  • each test piece 25 mm in diameter with the composition as shown in Table 2 was heated at a temperature of 870° C. to 980° C. for 1 hour in N 2 gas atmosphere, then water-quenched, tempered at a temperature of 200 to 700° C. for 2 hours, rapidly cooled in water, and measured for hardness.
  • the purpose of the test was to investigate and analyze the effect of each alloying element on the resistance to temper softening and to establish the way to design alloys that can have a tempered HRC hardness of 45 or higher by tempering at 600° C.
  • Mo, V, and W are accompanied by the maximum addition amounts YMo, YV, and YW, respectively. If any of these elements is added in an amount of more than the maximum addition amount, the hardness of the alloyed steel is calculated by using the maximum addition amount in place of the actual amount in the formula.
  • FIGS. 6 to 10 each show the result of the measurement of tempered hardness of different steel products (indicated by “Found”) shown in FIG. 2 and the result of the calculation of tempered hardness based on the process in which the effect of each alloying element on the tempered hardness is analyzed and quantified (indicated by “Calculated”). These graphs show that there is very good agreement between the calculated and found values of the tempered hardness of each alloyed steel and that the effect of each alloying element can almost reasonably be quantified.
  • FIG. 6 shows the found values (at a quenching temperature of 870° C.) and the calculated values of the tempered hardness of products Nos. 1 to 9 for comparison.
  • Nos. 1 to 3 show the effect of Mo or V addition on low Cr-high Si steels
  • Nos. 4 to 6 show the effect of V, Ni, and W
  • Nos. 7 to 9 show the effect of the combined addition of Al and Ni.
  • the drawing shows that Si, Mo, V, W, and Al each develop significant resistance to temper softening even at low Cr content and that particularly, the Al-induced resistance to temper softening is very well calculated at the same degree of influence as Si, and Al develops the resistance to temper softening by substantially the same mechanism as Si.
  • FIG. 7 shows the found values (at a quenching temperature of 870° C.) and the calculated values of the tempered hardness of products Nos. 10 to 22 for comparison.
  • product No. 16 undergoes the process in which quenching is performed at a higher temperature of 980° C. and then tempering is performed, and it shows that the process of dissolving 0.5% by weight of V into the alloy and then tempering effectively contributes to the resistance to temper softening.
  • FIG. 8 shows the found values (at a quenching temperature of 900° C.) and the calculated values of the tempered hardness of products Nos. 23 to 29 for comparison.
  • This shows the results of the investigation of the relationship between high content of Cr and Mo and V addition and demonstrates that Mo and V forms the above-described relationship even in the case that Cr coexists in an amount of about 3% by weight.
  • the product is highly hardened to have HRC55, which is equivalent to or higher than the 600° C. tempered hardness of hot work tool steel SKD6, for example.
  • HRC55 which is equivalent to or higher than the 600° C. tempered hardness of hot work tool steel SKD6, for example.
  • products Nos. 23 to 29 according to the present invention is apparently useful as a hot work tool steel.
  • These products have Cr and Mo contents reduced to less than 3% and less than 1% by weight, respectively, and are quenched at a reduced temperature of 900° C. and therefore more economical.
  • the carbon content of these steels is limited to at most 0.55% by weight. However, it will be understood that the carbon content is more preferably 0.45% by weight or less in terms of the content range of SKD6.
  • FIG. 9 shows the found values (at a quenching temperature of 950° C.) and the calculated values of the tempered hardness of products Nos. 30 to 33 for comparison.
  • the drawing shows the effect of higher Cr content than those in FIG. 8.
  • the result of analysis shows that the effect of Cr on the resistance to temper softening drastically decreases as the Cr content becomes about 3.5% by weight or more and that the steel with a Cr content of more than 3.5% by weight is drastically reduced in the Si-induced resistance to temper softening. Therefore, it is apparent that the Cr usage is preferably limited to 3.5% by weight or less so that Cr can be effective at developing the resistance to temper softening.
  • FIG. 10 shows the found values (at a quenching temperature of 900° C.) and the calculated values of the tempered hardness of products Nos. 34 to 38 for comparison.
  • the drawing shows the effect of Mo, W, and Si in a high Cr content range. It has been found, from the result, that the maximum effective addition amount of W is about 1.0% by weight at 900° C. and that the addition of W in an amount of more than 1% by weight drastically reduces the maximum effective addition amount of Mo, and therefore the addition amount of W is desirably not more than 1% by weight. It has also been found that 6% by weight or more of Cr further drastically reduces the Cr-induced resistance to temper softening (see No. 38 ).
  • the alloying elements should be used in such a combination that the following formula is satisfied.
  • Table 2 also shows the results of the 2U Charpy impact test (using JIS (Japanese Industrial Standards) No. 3 test piece) in which the steels tempered at 200° C. or 600° C. for 2 hours were examined.
  • Table 3 also shows the results on additional materials according to the present invention and comparative materials. It has been found, from the results, that among the materials tempered at 200° C. or 600° C. for 2 hours,
  • the materials with Al and Ni added in combination have high-toughness. It has also been found that thanks to the combined addition of Al and Ni, the upper limit of the carbon content can be about 1.2% by weight in the tempered martensite structure steel showing such toughness as a Charpy impact value of 5 kgf m/cm 2 or more. TABLE 3 Quenching TPNo. C Si Mn P S Ni Cr Mo V W B Nb Al ° C.
  • FIG. 11 shows the relationship between the hardness and the Charpy impact value on products Nos. 47 , 48 , and 49 each quenched from the temperature as shown in the table and tempered at each temperature of 200 to 500° C. for 3 hours. No temper brittleness-induced decrease in the impact value was observed between low-temperature tempered HRC60 and 500° C. tempered HRC47, and particularly, the toughness was rapidly recovered by tempering at 200° C.
  • FIG. 12 shows the relationship between each tempering temperature and the Charpy impact value on product No. 47 in Table 3 and products Nos. 10 and 12 in Table 2. It is apparent, from the drawing, that product No. 12 is rapidly embrittled by tempering at 200° C. or higher, but product No. 10 with a high Si content retains the toughness at 350° C. or lower and is significantly embrittled at 500° C. and recovers the toughness at 600° C. It is also apparent that product No. 47 with Al and Ni added in combination shows no temper embrittlement but very high toughness.
  • the example of the low-carbon Al—Ni steel (No. 40 ) is found to show excellent toughness even in the tempering process at a low temperature of 200° C. It is also apparent, from the comparison with the results on the steels in Table 3 (Nos. 39 , 41 , 45 , and 46 ), that the Al—Ni steel has a very high toughness in a wide range of carbon content and a wide range of hardness and that a suitable carbon content is preferably from 0.15 to 1.20% by weight. Therefore, it has been found that the alloy is preferably designed in such a manner that HRC45 or higher is established by tempering at 600° C. while wear resistance is retained at HRC45 or higher.
  • FIG. 14 showing the distribution of the hardness in the test pieces carburized, quenched, and tempered
  • the hardness of the surface carburized case is presented as a Vickers hardness from Hv750 to Hv800 (corresponding to HRC62 to HRC64), and the Charpy impact values of the respective test pieces are as follows: No. 39 : 1.74 kgf m/cm 2 ; No. 40 : 11.9 kgf m/cm 2 ; and No. 41 : 1.24 kgf m/cm 2 .
  • products Nos. 39 and 40 was found to keep the ⁇ Fe phase remaining. Therefore, a high temperature of 910° C. was substituted for 850° C.
  • FIGS. 15 ( a ) and 15 ( b ) are photographs each showing the structure with a depth of 0.2 mm from the surface of the Charpy test piece, which was prepared by carburizing product No. 40 at 1000° C. so as to provide a surface carbon content of 1.1% by weight (a) or 1.3% by weight (b), temporarily cooling it to the A1 temperature or less, re-heating it at 870° C. for quenching, and tempering it at 200° C. for 3 hours.
  • cementite particles with an average particle diameter of 1 ⁇ m or less are almost homogeneously dispersed, and the surface carburized case has an HRC hardness of 62.
  • gear members having a surface carburized case structure in which lots of fine cementite particles are dispersed have been excellent in contact pressure resistance and expected to form a more compact gear for reduction gears, but have been very poor in toughness. From the results of the Charpy impact test (No. 40 : 4 to 6 kgf m/cm 2 and No. 41 : 0.7 to 1.0 kgf m/cm 2 ), however, it has been found that the steel can improve in toughness by adding Al and Ni in combination according to the present invention and that such a steel with cementite particles dispersed can be used to form a high contact pressure-resistant gear.
  • wear resistance data of some conventional wear resistant components for construction or earth work machinery which are potential use of the above high-hardness, high-toughness steels, are organized to show the effect and the advantage of the present invention.
  • Table 4 shows typical components and their carbon content and their quenched and tempered hardness, and tempering parameters calculated from typical alloying constituents. Many of these components are designed to satisfy both high toughness and high hardness and therefore commonly contain 0.25 to 0.40% by weight of carbon. Such components are rarely used at a hardness of HRC52 or higher and therefore apparently insufficient in wear resistance.
  • Such steels include no case where Al and Ni are added in combination in order to provide high hardness and high toughness. Considering the results of the Charpy impact values in the above examples, such steels still have problems of cracking, chipping, and fracturing due to insufficient toughness.
  • the applicant has data concerning the relationship between the hardness of various steels and the gouging wear resistance, wherein the wear resistance (W: wear amount) of the quenched and tempered steel with a Vickers hardness of Hv500 is normalized as 1, and reduction in hardness by friction heating is not significant.
  • W wear resistance
  • the high-hardness, high-toughness steel with Al and Ni added in combination can be used and appropriately heat-treated to form a significantly improved-wear resistance crawler link, crawler shoe, crawler bush, bucket tooth, cutting edge, end bit, segment teeth, bottom tracker roller, tunneling tool bit, shank, disk cutter, chisel tool, or soil cutter for earth stirring in a soil-improvement machine, each having a hardness of HRC55 or higher and a Charpy impact value of 5 kgf m/cm 2 or more.
  • the bucket tooth, ripper point, end bit, and cutting edges which are frequently used in excavating rock and need resistance to temper softening, can be prevented from cracking, chipping, or fracturing by using the above tempering parameters, appropriately adding the alloying elements so as to provide a hardness of HRC45 or higher, preferably HRC50 or higher by tempering at 600° C., and enhancing the toughness by the combined addition of Al and Ni.
  • Example 4 and FIG. 11 show that product No. 40 in Table 3 ensures a high Charpy impact value at a hardness of HRC60.
  • Product No. 40 has a very high toughness (see Example 5) in contrast to the carburized, quenched and tempered SCM420H product, which is supposed to form gears. Therefore, it is apparent that the inventive steel with Al and Ni added in combination and 0.45 to 1.2% by weight of carbon can be used and worked into a gear shape and then induction-quenched and tempered or subjected to known appropriate quenching and tempering to form a gear with a surface hardness of HRC55 or higher at lower cost than that for conventional carburized, quenched and tempered gears.
  • the surface hardness is preferably HRC58 or higher in terms of improvement in contact pressure resistance strength of the gear and that the cementite particles with an average particle diameter of 1 ⁇ m or less are preferably dispersed in the surface layer.
  • the amount of P is limited to 0.01% by weight or less because P can significantly cause grain boundary embrittlement and significantly facilitate the weld cracking, the amount of B is carefully controlled in order to ensure the hardenability of the steels, and the carbon content is limited to between 0.1 and 0.3% by weight.
  • the bucket wear-resistible steel plate in Table 4 which is to be fillet-welded to the bottom of the bucket, has a limited carbon content of 0.3% by weight or less and comprises controlled constituents according to the above to be free from weld cracking. Therefore, such a steel can be insufficient in wear resistance.

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US20050241734A1 (en) * 2002-08-21 2005-11-03 Komatsu Ltd. Rolling elements
US20050284258A1 (en) * 2002-12-27 2005-12-29 Komatsu Ltd. Wear-resistant sintered contact material, wear-resistant sintered composite contact component and method of producing the same
US20070237590A1 (en) * 2006-03-15 2007-10-11 Al-Hussain Mariam J S Rotary tool
ES2293837A1 (es) * 2006-07-31 2008-03-16 Sidenor Industrial, S.L. Proceso de fabricacion de un acero, y acero obtenido en este proceso.
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